Phosphogypsum is a by-product of processing phosphate rock into phosphoric acid fertilizer. The production of 1 ton of phosphoric acid generates approximately 4 to 5 tonnes of phosphogypsum. Phosphogypsum is essentially a waste product. Phosphogypsum may have low level radioactivity which prevents its use in various applications.
Phosphogypsum is typically stored by being slurried and piled into large stacks, which can be up to hundreds of feet high, in open air storage sites. Water percolating through the stacks forms ponds. In 2005, there were 24 phosphogypsum stacks in Florida alone, containing 1.2 billion tonnes of phosphogypsum and 50 billion gallons of pond water (Perpich et al, 2005).
In active phosphoric acid fertilizer plants, such ponds are typically used as reservoirs for process water for use in a closed loop. The pond water is toxic and needs to be treated before it can be discharged. Furthermore, closed stacks continue to produce a contaminant-containing leachate requiring treatment.
Pond water associated with phosphogypsum stacks is strongly acidic and contains numerous contaminants including large amounts of phosphates. Data collected from a number of sources are summarized in Table 1. The column headed “Representative Value” contains results from a composite of 18 samples from 6 different plants representing the composition of saturated fresh pond water (Kennedy et al., 1991). The reported phosphorus concentration of 6,600 ppm as P is equivalent to 20,220 ppm PO4 or 0.22 Mol/L. Pond water also contains significant amounts of ammonia (ammonia is often added to phosphoric acid in phosphoric acid plants to make di-ammonium phosphate) and magnesium.
TABLE 1Typical composition of pond waterComponentUnitsRangeRepresentative ValuepH1.3-3.01.55ConductivityμS/cm15,000-30,000Ammonia (as N)ppm 500-2,000592Calciumppm 500-3,0001,155Chlorideppm 10-300Fluorideppm  200-15,0007,600Ironppm 5-300216Magnesiumppm200-500286Phosphorus (as P)ppm  500-12,0006,600Potassiumppm100-400276Silica (as Si)ppm 100-4,0001,910Sodiumppm1,000-3,0001,995Sulfate (as S)ppm1,000-4,0001,695
Pond water treatment chemistry is relatively complex. Pond water may contain ten major components that can form numerous soluble species and precipitates when the pH is changed and cations are added. As indicated by the data under the column headed “Range” in Table 1, the composition of pond water can vary significantly.
A cost-effective and efficient process for treating phosphate-containing wastewater while recovering commercially useful phosphate compounds would be desirable.